The numbers of others are variable depending on your empire. As you build more ships and colonise more worlds others will pop up. I don't know what if any the maximum number is set at. The percentage modifiers for each race will influnece how quickly new characters appear as well.

AFAIK there is no limit to the number of characters that will show up in game (save that there are pre-defined characters in the character file and you use them all). As Ralzakark pointed out, the rate at which they appear will vary from game to game and from race to race.

Characters show up randomly (via a random number generator and the modifier that is in the races file) through out the game. I can't explain exactly how it works (since I am no coder), but the if I were to wager a guess, I'd say the general idea is that you hit the target number for a 'generate character event' and then another 'roll' is done to determine what type of character, then if it is a pre-defined character, they appear in the order listed, or a random character will appear if no characters are defined.

Some one else (or Elliotg or Erik) can probably explain much better how it works and why it works the way it does.

Wow Shark and Ed I had no idea. I figured that there must be some sort of limit which I had just never reached.

On a vaguely related note - does anybody know if 'inspiring' and 'demoralising' work as advertised in the modding guide/pop-up windows?

I've never really been able to see any effect from 'inspiring' on my leader/main colony governor (but admit that this is far from clear as various increases happen from time to time). In my last game I had a scientist with 'inspiring' and although he was kept company by one or more scientists for the entire game I never saw any increase to skills which I would not have otherwise expected. This game I had my second scientist pop with 'demoralising' and although he's keeping my main lightbulber company I've not seen any decrease in skills (or even any noticeable decrease in the rate of skills increase). Am I kidding myself or do these traits actually not have any in game effect?

Yep, you can make the game more challenging, or easier, by messing with the characters - although making it easier is, well, easier. In vanilla each race gets a good character to start with. For Ugnari it's a spy, humans a diplomat, etc. Taking that away can make it harder but it also alters how each race plays

Even more difficulty altering can be done by assigning a trait to all characters of a type. Making all your characters have, say, 'obnoxious' and 'lazy' can create some extra difficulty.

Best way to do it is to follow the modding guide and make a theme in which you do all your customisation. Beware of dropping comma's... Other than that it is quite easy.

Inspiring and Demoralizing work in a way that is not really evident. You will notice other characters at the same location will gain skills faster or slower if these types of characters are also there. It does not prevent them from gaining skills or speed it up to 'light speed' gains, just has a constant effect.

If I were to guess, the modifier would be a +10% or -10% effect on skill up speed.